An electronic balance is used to detect a very small change in the mass (or the weight) of an object, and is especially useful for a thermal analysis of a sample in which the change in the mass of the sample is continuously detected while it is heated (or cooled). Such an electronic balance is explained referring to FIG. 1. A sample tray 9 and a feedback coil 7 are supported by a beam 4 which is sustained by a fulcrum 8 placed between them, and a shutter 3 is fixed to an end of the beam 4. Apart from the beam 4, a photoemitter 2 and a photoreceiver 1 are placed opposing each other with the shutter 3 between them. The photoreceiver 1 is composed of two photosensitive areas 11 and 12 aligned along the direction of the movement of the shutter 3 (vertically in the case of FIG. 1). When the mass of a sample 15 on the sample tray 9 changes, the beam 4 turns and the shutter 3 moves vertically with its shadow 13 on the photoreceiver 1. This changes the shares of the light received by the two photosensitive areas 11 and 12, and thus the difference between the outputs of the two photosensitive areas 11 and 12 changes, too.
A magnet 6 is placed around the feedback coil 7 so that they react magnetically. The position of the feedback coil 7 can be changed by altering the electric current supplied to the feedback coil 7, and a controller 5 controls the current so that the difference of the outputs of the two photosensitive areas 11 and 12 does not change (i.e., so that the beam 4 is stable). Thus the mass of the sample 15 is measured by detecting the current flowing through the feedback coil 7.
The turn of the beam 4 is detected by an imbalance detecting circuit, as shown in FIG. 5, provided in the controller 5. In the circuit diagram of FIG. 5, the photodiodes D11 and D12 correspond to the photosensitive areas 11 and 12, respectively. The current I1 flowing through the photodiode D11 represents the amount of light received by the photosensitive area 11 and the current I2 flowing through the photodiode D12 represents the amount of light received by the photosensitive area 12. A subtracting circuit composed of an operational amplifier OP3 and three resistors R11, R12 and R2 produces a voltage Vdt3 corresponding to the difference between the two currents I1 and I2. The current to the feedback coil 7 is controlled by another circuit in the controller 5 so that the voltage Vdt3 is constant.
A problem about the prior art electronic balance is that, a drift occurs in the outputs of the photoreceiver 1 and also in the difference of the outputs due to change in the ambient temperature or other factors. In this case, the imbalance detecting circuit (FIG. 5) of the controller 5 generates an erroneous signal though the beam 4 does not turn, and the controller 5 alters the current to the feedback coil 7, which turns the beam 4, whereby a wrong mass is measured.